1. Field of Technology
The present invention generally relates to the art of vapor deposition of metal in a vacuum and, more particularly, to a vapor masking device for use during the execution of a sputtering technique for forming a thin film of aluminum on discs of a kind generally known as a "Compact Disc".
2. Description of the Prior Art
Referring to FIG. 2, a compact disc D, now commercially available as a sophisticated product which may be regarded as a future substitute for the phonograph record, has outer and inner peripheral margins OM and IM which are non-recorded regions both coaxial with the center hole defined therein, a recorded region being located between these peripheral margins OM and IM. When it comes to the manufacture of the compact disc, the outer and inner peripheral regions must be masked from vapor of aluminum during the execution of a sputtering technique so that a thin film of aluminum can be deposited only on the region intermediate between the outer and inner peripheral regions.
One type of masking device hitcherto used for masking the outer and inner peripheral regions of the disc is disclosed in, for example, Japanese Laid-open Utility Model Publication No. 57-173134, published Nov. 1, 1982. According to this publication, the masking device is shown as comprising a support plate having defined therein an annular recess of a shape completely complemental to the shape of the disc and of a depth equal to the thickness thereof. This support plate includes a lid hingedly mounted on the support plate and having defined therein a circular opening of a diameter required to shield the outer peripheral region of the disc placed inside the annular recess.
This masking device also comprises a center piece having a radially outwardly extending annular flange and adapted to be removably threaded to the center mount of the support plate located radially inwardly of the annular recess, said center piece being a member separate from the support plate.
When in use, the disc is mounted in the annular recess, and the lid is then pivoted to a closed position. While only one of the opposite surfaces of the disc so mounted is exposed to the outside through the opening in the lid with the outer peripheral region of the disc concealed by the peripheral lip area of the lid around the opening, the center piece is then threaded to the center mount to permit the annular flange to conceal the inner peripheral region of the disc.
Another type of masking device also currently in use is depicted in FIG. 4. The prior art masking device so far shown includes four marking units each being generally similar in design to that disclosed in the previously mentioned publication, but differing therefrom in that the lid 22 is separate from the support plate and also in that the center piece 21 is carried by the lid 22 by means of a pair of generally U-shaped connecting bridges 23. The pair of the connecting bridges 23, each made of a slender metal rod, straddle over an annular opening between the lid 22 and the center piece 21 while extending radially in opposite directions with respect to each other, thereby retaining the center piece 21 in position within the opening in the lid 22 in coaxial relationship therewith.
In designing the masking device shown in FIG. 4, each of the connecting bridges with its opposite ends rigid with the lid 22 and the center piece 21 must protrude laterally outwardly from both the lid 22 and the center piece 21 a distance great enough to avoid the possibility that the thin film of aluminum eventually formed on the surface of the disc by the use of the sputtering technique will bear markings of the connecting bridges 23. Otherwise, the connecting bridges 23 will provide an obstruction to the deposition of the aluminum vapor on the surface of the disc with the thickness of the aluminum thin film consequently rendered irregular. The necessity of the relatively great distance over which the connecting bridges protrude laterally outwardly makes the masking device as a whole bulky in size.